Ten percent of American children have Class II malocclusion ("buck teeth"), and nearly half of all orthodontic patients are treated for a problem of this type. The cause usually is disproportionate jaw growth, and the best treatment approach remains unknown. The choices are early treatment (prior to adolescence) to try to modify growth and correct the jaw relationship, or later treatment (in adolescence) to reposition the teeth to compensate for the incorrect jaw position. The important questions are whether growth modification can be accomplished, and if so, whether it makes any difference to subsequent treatment and outcomes. A prospective randomized clinical trial (RCT), which we now have in progress to compare headgear and functional appliance methods of early treatment versus no early treatment, is necessary to obtain unbiased answers. We also are addressing additional questions and a broader coverage of age groups through a larger retrospective-prospective observational sample of Class II patients. At this point, the 150 children specified initially have been enrolled in the RCT; 107 have finished phase 1 and the remainder will do so by the end of the current period. The preliminary data indicate that both types of early treatment affect growth. All these children require phase 2 treatment, which has begun for 68 patients but has been completed so far for only two. A major goal of the project now is to complete phase 2 treatment for RCT patients, comparing the outcomes at the end of phase 2 for those who had early treatment versus those who did not. Long-term outcomes are greatly affected by how posttreatment retention is managed, and in the RCT patients, we will compare the effectiveness of conventional uni-arch orthodontic retainers and a modified functional appliance as a retainer. Finally, using the larger observational group to obtain broader patient coverage for comparison with the RCT results and to provide data on questions that require a longer time span, we will evaluate the effectiveness of Class II treatment and the posttreatment skeletal and dental changes following treatment. No similar study has ever been undertaken. The data will clarify the best approach to these problems, and will be important nationally and internationally in determining treatment modalities.